Morphometry of wrinkle ridges on Venus: Comparison with other planets

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E5, PAGES 11,103-11,111, MAY 25, 1998 Morphometry of wrinkle ridges on Venus: Comparison with other planets M.A. Kreslavsky Kharkov Astronomical Observatory, Kharkov, Ukraine A.T. Basilevsky 2 Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow Abstract. The widths of wrinkle ridges on Venus and the spacing of their network were measured at 30 randomly selected sites on the Venusian plains. At one of the sites, heights of ridges were also estimated. Results of our measurements indicate that (1) wrinkle ridges on Venus form a quasi-periodic rather than chaotic network; (2) there are regional variations of the typical spacing within the network, with the mean for all 30 sites being 12.9 km, and the standar deviation of the means of all sites being 5.8 km; (3) there are regional variations of typical ridge width with the mean width being 1.25 km, and the standardeviation of the means of all sites being 0.66 km; (4) there is no site-to-site correlation between the mean ridge widths and spacings. Comparison with other terrestrial planets shows that wrinkle ridges on Venus are generally narrower than on the other planets, and the spacing of the ridge network on Venus is smaller than on Tharsis Plateau, Mars. Global surface shortening due to formation of the wrinkle ridge network is estimated to be of order of 0.1%. 1. Introduction Providence, Rhode Island. Copyright 1998 by the American Geophysical Union. Paper number 98JE /98/98 JE evidence for similar proposed origins [P!escia and Golombek, 1986; Watters, 1991]. Early hot debates on how wrinkle ridges formed, and whether they are purely volcanic, purely Wrinkle ridges are known on the Moon, Mars, Mercury, and Venus. On the Moon, where they were first found, they tectonic, or volcanic-tectonic features (see summaries by often form concentric patterns in the nearside maria [e.g., Sharpton and Head [1986], Plescia and Golombek [1986], Strom, 1972; Fagin et al., 1978; Raitala, 1978; Solomon and and Watters [1988]), have now calmed down. Wrinkle ridges Head, 1979; Wilhelms, 1987]. On Mercury, only a few are considered in recent publications only as the result of features of that sort are known [Strom et al., 1975]. In some compressional deformation [e.g., Plescia, 1991; Watters, regions on Mars wrinkle ridges are numerous [e.g., Lucchitta 1991; McGill, 1993; Allemand and Thomas, 1995; Bilotti and and Klockenbrinck, 1981 ]. Plescia and Golombek [ 1986] and Suppe, 1997; Watters and Robinson, 1997] although there is Watters [1991] considered also some terrestrial tectonic no general agreement on the concrete mechanism of the features as analogs of wrinkle ridges on other planets. One of deformation. Two scenarios were proposed: (1) folding the best documented eamples of the terrestrial morphological followed by faulting [e.g., Watters, 1988; Watters and analogs of the wrinkle ridges of the Moon, Mercury and Mars Robinson, 1997], and (2) faulting followed by folding [e.g., are thrust faults formed in October 1968 as a result of 6.9 M Plescia and Golombek, 1986]. magnitude Meckering earthquake in Western Australia It is commonly thought that wrinkle ridges are [Gordon and Lewis, 1980]. On Venus, wrinkle ridges were compressional tectonic features formed in a strong neardiscovered in Magellan radar images [e.g., Solomon et al., surface layer above a mechanically weaker material. For the 1992, McGill, 1993] and turned out to be more abundant than Moon, Mars, and Mercury, basalt filling of maria and other on other planets. They are common features on the vast types of volcanic plains is considered to play the role of the regional plains, which are dominant on Venus. According to strong layer above weaker megaregolith substrate formed in Bilotti and Suppe [1997], about 43% of the plains have a the heavy bombardment period. Preeisting zones of well-developed network of wrinkle ridges. Basilevsky and weakness, such as buried craters [Allemand and Thomas, Head [1995a,b] estimated total abundance of plains with 1995], faults, and buried topography, are important in the wrinkle ridges on Venus as about 70% of the planet surface. localization of the ridge-forming deformation, while buried The morphology of wrinkle ridges is strikingly similar on regolith and pyroclastic layers might act as a decollement all terrestrial planets. This fact is believed to be the main [Sharpton and Head, 1982, 1986]. On Venus there is no reason to suspect the eistence of impact-produced 1Also at Department of Geoscience and Astronomy, University of Oulu, Finland. megaregolith immediately beneath the observed basaltic 2Also at Department of Geological Sciences, Brown University, plains; however highly tectonized tessera material which is 11,103 believed to underlie many of the Venusian plains [Ivanov and Head, 1996] may play the role of weaker substrate. Also it cannot be ecluded that deformation of a strong layer overlying mechanically weaker material is not a requirement for wrinkle ridge formation at all. For eample, the surface

2 11,104 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS.2.0 krn I... :... ::..'2' ' :...:.i: =:... I i I - ß Figure 1. Magellan radar image of a circular feature formed by wrinkle ridges (fragment of C1-MIDR 60N153;1). The feature is located at 66øN, 164øE. Feature diameter is about 60 km. Aeolian features associated with wrinkle ridges (arrows) are clearly seen. features of the Meckering thrust faults, showing striking morphologic similarity to the wrinkle ridges of the Moon, Mercury, Mars, and Venus, were formed in the surface layer composed of soil, alluvium, and weathered granites and gneisses overlying less weathered and mechanically stronger rocks [Gordon and Lewis, 1980]. On the Moon, wrinkle ridges often trace irregularities of the basalt layer / megaregolith interface; for eample, in subsequent basalt flow deposits, they outline inner rings of mare basins and some other buried impact structures [e.g., Strom, 1972; Fagin et al., 1978; Raitala, 1978; Solomon and Head, 1980]. Figure 1 shows a similar eample for Venus. At this site the circle formed by wrinkle ridges outlines a probable large impact crater buried by the plains material. In some regions on Mars, wrinkle ridges form a regional network [Watters, 1991 ], similar to occurrences on Venus. Several publications describe key morphometric characteristcics of wrinkle ridges of the Moon, Mercury, and Mars: Watters [1988] measured widths and heights of 185 wrinkle-ridge-related features on the Moon, Mars, and Mercury ranging from rather steep ridges (with typical width/height ratio < 10) to arches (width/height -- 70). Using typical width, Watters [1988] subdivided ridges into firstorder (the widest), second-order, and third-order ridges. Profiles of wrinkle ridges in Lunae Planum on Mars were studied also by Plescia [1991]. Watters [1991] measured frequency distributions of distances between wrinkle ridges for 15 areas on the Tharsis Plateau on Mars, where the ridges form quasi-periodical (in the sense of dominant ridge-to-ridge distances) regional systems. Allemand and Thomas [1995] also made mesurements of the spacing in that region. We could not find in literature systematic studies of these morphometri characteristics for wrinme ridges of Venus. The lack of them is a reason for the present study. This paper is devoted to measurements of three key characteristics of Venusian wrinkle ridges: their spacings, widths, and heights. We succeeded in measuring the wrinkle ridge spacings and widths in 30 different regions of Venus (see early results of Kreslavsky and Basilevsky [1997]) and estimated regional and planetwide means and standard deviations of these parameters. We tried to measure also the wrinme ridge heights applying different techniques but failed

3 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS 11,105 in all cases but one and concluded that it is not possible with the available data. We compare the results of our measurements with the published characteristics of wrinkle ridges of the Moon, Mercury, and Mars. Finally, on the basis of measured characteristics of Venusian wrinkle ridges we try to assess regional strain due to ridge formation. 2. Measurements and Results 2.1. Spacing To study the spacing of wrinkle ridges on the plains of Venus, we selected 30 sites. Criteria for the selection were as following: (1) wrinkle ridges should be present in the site, (2) F-MIDRP images should be available, (3) the sites should be distributed more or less evenly (in the sense of number of sites per area unit) in latitude and longitude belts. As a result of this approach, the selected sites are distributed nearly randomly both in the general sense and in the sense of distance to such key elements of Venusian geology as massifs of tessera terrain, ridge belts, rift zones, and large volcanoes 2.3. Height (Figure 2). For each site we plotted on the image (only F- MIDRPs were used) a line segment equivalent to 200-km length in the direction normal to the dominant orientation of wrinkle ridges at the site. We measured distances between intersections of neighboring ridges with this segment. This technique is similar to the technique used by Watters [1991] in his measurements of wrinkle ridges spacing on Mars. From 8 to 38 ridge-to-ridge distances were measured for different sites. The mean and the standard deviation of the spacings for each of 30 sites are presented in Table 1. The mean spacing for all 30 sites, estimated as an average of the means of all sites, is 12.9 km, with the standard deviation of the means of all sites being 5.8 km Width three features visible there are just individual wrinkle ridges, the measurements results are meaningful. Central lineament is For each of our 30 sites we estimated the width of the ridges at 9 to 18 points in an area of about 50 km around the spacing-measurement line segment. For 'most of ridges, the break in slope at the base of the ridge (foot) for both sides of the ridge is clearly seen on the images, and the estimation of width is not a problem. However, for some ridges it is not easy to identify the foot of the ridge on the images. In many higher by the measured value, which puts a constraint on the wrinkle ridge height. Another method of height estimation is based on the difference in apparent lengthening and shortening of slopes at different illumination geometry [e.g., Connors, 1995]. For points B and C (Figure 4), where ridges are about 2.5 km wide, this method results in height limits of m and cases all the ridge surface from foot to foot looks somewhat m, respectively. brighter than the surrounding plains, and the ends of this brightening mark feature edges. Special attention was paid to In general, measurements based on radar stereo viewing, as was done for point A (Figure 4), are rarely possible, because distinguish these brightened ridges from bright, supposedly at the 110 to 250-m Magellan image resolution [Ford and aeolian features [Greeley et al., 1992] sometimes associated Plaut, 1993] it is too difficult to identify with the necessary with ridges. A typical eample of such features is shown in accuracy (on images made at the opposite radar viewing Figure 1. Site C in Figure 3 is an eample of a less obvious directions) the same features on the ridge tops. Measurements case. Bright patches are probably similar aeolian features. based on the apparent slope lengthening and shortening, as However, they also might be caused by increased surface was done for points B and C (Figure 4), are of very low roughness due to small-scale surface disruption during ridge formation. In any case, in our measurements we did not accuracy. The apparent widths of shortened slopes facing toward the radar are usually under the resolution limit, so only include features like this as ridges or parts of ridges. In some cases, neither the bases of the ridge nor the brightened ridge surface was seen in the radar images. In these cases what is seen is probably the ridge crest [McGill, 1993]. The supposed crests are sinuous at small scale, probably because they wander from one foot of the unseen arch to the other, a situation that is typical for ridges on other planets [Watters, 1988]. Eamples are shown in Figure 3, sites A and B. In these cases we estimated the ridge widths by using the characteristic width of this small-scale wandering. Finally, in some cases there is no small-scale sinuosity, and wrinkle ridges are seen in radar image as lineaments whose width is apparently smaller than the image resolution. In these cases we assumed that the ridge width was 230 m which is the lowest resolution of the images used in this analysis. In general, the number of cases where the width of the ridges was clearly seen and measured without any assumptions was significantly larger than the number of cases where some assumptions were involved (presence of unseen arch, completely bright lineaments, ridge width smaller than that the image resolution limit). Hence the measurement results are not seriously dependent on the assumptions. The results of the measurements are listed in Table 1. The mean width for all 30 sites, estimated as average of the means of all sites, is 1.25 km, and the standard deviation of the means of all sites is 0.66 km. We attempted to estimate the height of wrinkle ridges, and the only place where we achieved success was the site at 20.5øS, 161.0øE (Figure 4). The images for the first three cycles of the Magellan survey, that is, for three different viewing geometries, are available for this site, and ridges are rather wide, which is important for the measurements. At point A (Figure 4) the central bright feature of the 1.8- km-wide ridge is apparently its crest, and the eastern and western features are probably the break in slope at the base of the arch, possibly complicated with secondary ridges. Stereo analysis [e.g., Connors, 1995] gives 40- to 90-m limits for the height difference between the crest and the base. The eastern foot is m higher than the western one. Even if our interpretation of the situation in point A is wrong and the an upper limit of the apparent widths can be estimated. Margins of lengthened slopes facing in the opposite direction are difficult to identify accurately. Application of the subresolution clinometry technique [Kreslavsky and Vdovichenko, 1997] is difficult for several reasons. First, small-scale variations in radar brightness are significant, so it is almost impossible to find good areas for averaging data in order to reduce the noise. Second, the technique requires a

4 11,106 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS O '" 0 O O o C) q i

5 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS 11,107 Table 1. Results of Measurements of Wrinkle Ridges Spacings and Widths in 30 Sites on Venus Site coordinates, deg Spacing, km Width, km Latitude Longitude Mean S.D. Mean S.D. N Strain, % O Here S.D. is standard deviation, N is number of individual measurements. priori assumption of the ridge profile which cannot be reliably done. Of course, the topography of ridges whose width cannot be resolved cannot be estimated from radar images. Thus a global systematic survey of wrinkle ridge heights based on the available data seems to be impossible. eplanation for this regularity is the eistence of a predominant wavelength of strong surface layer deformation under single-ais compression [Watters, 1991 ]. A strong correlation between the mean spacing and its standard deviation is seen in Figure 5. The variance (standard deviation referred to mean values) values are almost the same for wrinkle ridges of Venus and Mars. Although we do not 3. Comparison With Other Planets and understand whether it is the additional evidence for the Discussion 3.1. Spacing Mean values and standard deviations of wrinkle ridge spacing for our 30 sites on Venus are compared with similar data of Watters [1991] for the Tharsis Plateau on Mars in Figure 5. If parallel ridges were distributed purely randomly, the similarity of wrinkle ridge formation mechanism on these two planets or not, we feel the necessity to report this as the characteristics of the phenomenon. It is clearly seen (Figures 5 and 6) that spacing of ridges on Venus is systematically smaller than on Mars. The global average is about three times smaller for Venus than for the Tharsis Planum. The influence of the difference in gravity on Mars and Venus on spacing of wrinkle ridges can be ridge-to-nearest-ridge distance would have an eponential evaluated using elastic and viscous models considered by distribution. In this case the standard deviation of the distance Watters [ 1991]. Both models assume a layered structure with should be equal to the mean [Boas, 1983]. Our measurements strong upper layer(s) and weak substrate. For the elastic show that for each site the standard deviation of wrinkle ridge spacing is significantly smaller than the typical spacing value (Table 1). This demonstrates that wrinkle ridges tend to form a more or less regular network. The same situation occurs on the Tharsis Plateau on Mars and is eplained by Watters [1991] as a result of control of the dominant wavelength of buckling (which Watters assumes to be the deformation mechanism) by the deformed layer thickness. A probable model, spacing is approimately proportional to g-l/4 (g is acceleration due to gravity). For the viscous model, depending on the set of other parameters, the influence of gravity changes from proportionality to g-m to absence of any dependence. Figure 6 shows how the Venus data can be scaled to Martian gravity using the strongest dependence (_g-m). It is clearly seen that the gravity scaling cannot account for all the difference in spacing. The most probable

6 :.,.. 11,108 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS A B the resolution limit of Magellan images. Apparently, some ridges are narrower than the limit. Our measurementshow that wrinkle ridges are generally narrower on Venus than on other planets. Comparison of the variations of average width from site to site with the standard deviation of width for each site shows that there are regional variations of the typical width of wrinkle ridges. The global averages of both spacing and width of wrinkle ridges on Venus are smaller that the global averages of the same parameters for Mars. It is interesting to note that the relative decrease of both of these parameters, if we compare Mars and Venus, is about of the same order of magnitude. We do not fully understand a reason for that but, as in the case with similarities in the spacing variance values, we would like to report this as a characteristic of the phenomenon. It is surprising that among our 30 randomly chosen sites on Venus there is no correlation between average width and spacing (Figure 8). This correlation would be epected if both.:. J... ::i;;i!i 4-:' ½---:':::. ';.:'... ::' *...:?:...{:-% ::.:.:... -{:. ";5:: ;;... ;.-;:.:...{'-; :.: -... ::., ;.;.-:,.::. :" ::..... ß... :{::: ,:,. Figure 3. Magellan radar image of wrinkle ridges at 44.0øN, 1.0øE (fragment of F-MIDR 45N004;1). At sites A and B the radar-bright ridge crest apparently wanders from one foot of the supposed arch to the other. Bright features like that at site C are apparently of aeolian nature but may be related to smallscale surface disruption during ridge formation. cause for the difference is that the upper strong deforming layer on Venus is generally thinner than on Mars. Comparison of variation of the mean spacing from site to site with the standard deviation of spacing for each site demonstrates that the variation of mean spacing reflects regional variations of the typical network spacing rather than measurement ambiguity. These variations could reflect the variations of the upper strong layer thickness at the moment of wrinkle ridge formation. -.::;. : :..,f.:: : *:-:-. ½:: : ß...:..;.::.:. '::'... ":.: '.... :}.'i½ L. ['-... ß...-..:;e. '-... '*.::::::::::::::::::::::::::::... ; :;... : ;:: ::::.. :, ', ½....;:. : " ; :- ; "½: :.t:*' '. :: :- :. "-..'*: :... ::, ;? ; ' ; : '"" :,,:.... :,:;:e....½..:,5:. ½ :..:......: : :: % ''..:...}:.:......,.:-..?;::;'::'.... ;? : ";Sf:i':;:::';;;.: ': {'; ;' :*. :.: r ::::...;-..:;-:::... ':'"-' 4: ' :... :?: ::e:; : ; ½%%M* 4:. :;:.... ::'e.:4:: ;:- -%t:' -. :' : ':':" ': ::: :.,...:.: -." :;-::-:. :' :':...::..;::..::::.... I.:½. -.:.½-...; :.5":' :.. %.. :? ::':-... :.:.:.. ';:::;'. ½:... :;½;?... ß.' 6 ' k... m...?::( ':'":'" :'....,. ::":'; "':: :? : :... : ß...--:- ' ' ;';i:". :::;. :' ß :*::::: ;] :;.... :t I... ;'; :':;:'; :: :;... "I... :t.. " ;.--f:. : :?(::.:..::: Width '.' :,,...:. Typical widths of wrinkle ridges on Venus are compared with Watters, [ 1988] data on the Moon, Mars and Mercury in Figure 7. The morphology of the ridges analyzed on Venus is similar to arches and first-order ridges and may sometimes correspond to second-orderidges on the other planets. Note that the lower limit of the width on Venus is determined by... ß...,..::.:... :.:.:':...-;:....'.;.... Figure 4. Wrinkle ridge at 20.5øS, 161.0øE (fragment of F- MIDR 20N162;1, cycle 1 of the Magellan survey). Marked are points where estimates of the ridge height were made (see tet). At points B and C the fight (eastern) foot of the ridge is not seen, but it can be traced on the Cycle 2 image.

7 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS 11, i i i i i i i ij i i e Moon i i i i i i Arches Venus, this study 2nd-order Ridges 1st-order r - IVlam, V em (1991) ' J L J ury Ridges r, 1 i v j Arches 2nd-order Ridges 1st-order [ ; ] w j w j A VenL/S r - 1 L J I I I I I I IJ I I I I I I i 10 Mean sparing, krn 100 Figure 5. Correlation diagram for mean value and standard deviation of distances between wrinkle ridges for 30 random sites on Venus (triangles) and 15 areas in Tharsis Plateau, Mars (crosses). Data for Tharsis Plateau are taken from Watters[ 1991 ]. o.1 I Width, km lo Figure 7. Comparison of wrinkle ridges widths on Venus (N = 337, where N is number of measurements) with data on the Moon (N = 74), Mercury (N = 14) and Mars (N = 88). Horizontal lines shows the range from minimum to maimum width. Circles indicate average values; brackets, average +/- standard deviation. Data for the Moon, Mercury, and Mars are taken from Watters[ 1988]. wrinkle ridge width and spacing are dependent on the upper strong layer thickness only. The absence of this correlation probably means that the typical width of the ridges is determined by both the thickness of the upper strong layer and the value of the regional ridge-forming strain. Moreover, the thickness and the strain are negatively correlated. ff so, the aforementioned difference between Venus and the Tharsis Plateau on Mars is due to the general difference in the thickness, while regional variations of the width on Venus are due primarily to variations in regional strain Height Our approimate estimations of ridge height show that height/width ratio for wrinkle ridges on Venus is of the same order as on other planets. This is additional evidence for the similarity of origin for wrinkle ridges on Venus and the other planets. Comparison of our estimates with Watters [1988] data is illustrated in Figure 9. I I I I I i I I I I J I I - Venus 1 I _ Mars, Tharsis Plateau - I I I I I I I I I I i i 10 Spacing, krn 100 Figure 6. Comparison of wrinkle ridge network spacing on Venus and in the Tharsis Plateau (Mars). Horizontal lines shows the range from minimal to maimal width. Circles indicate average values; brackets, average +/- standard deviation. Data for Tharsis Plateau are taken from Watters [1991]. Dashed lines show how the spacing for Venus could be scaled to Martian gravity using the strongest model dependence (~ g-u2) I I I I I, I I I 5 10 Spacing, km 30 Figure 8. Correlation diagram for the mean width and mean spacing of wrinkle ridges for 30 random sites on Venus.

8 11,110 KRESLAVSKY AND BASILEVSKY: WRINKLE RIDGES ON VENUS looo o / o,b o /o/ E. ø : o o / ß { o / 10, m 0.1 I dth, km 10 Moon i ure g. Heish width co.elation diagram for wrinkle on s, ercu, and the oon, compiled from []988] d our estimations for three wd le ridses on Venus. 4. Assessment of Regional Strain Mars Mercury Venus According to recent publications [e.g., Plescia, 1991; Watters, 1991; McGill, 1993; Allemand and Thomas, 1995; Bilotti and Suppe, 1997; Watters and Robinson, 1997], formation of a regional network of wrinkle ridges means regional one-ais shortening of the surface in a direction normal to the dominant ridge orientation. The relative regional shortening (strain), œ, can be estimated as the typical absolute horizontal shortening due to the formation of an individual ridge referred to the typical ridge spacing l. Because the mechanism of wrinkle ridge formation is not well known, it is impossible to estimate this shortening with great accuracy. If we neglect the faulting in the formation of the ridge and consider folding only, the horizontal shortening can be estimated as geometric lengthening of the surface profile [Watters, 1988; Plescia, 1991]. This lengthening is scaled by the ridge height h and depends on the ridge profile. We can epress the lengthening as Ch, where C is a dimensionless coefficient depending on the ridge profile. Because we could not make systematic measurements of the Venusian ridge heights, we use the data for Martian ridges, where morphology is strikingly similar to that of the Venusian ridges. A few aforementioned measurements of the Venusian ridges do not contradict this approach. Measurements of Martian ridge profiles by Plescia [ 1991 ] led to values of C in the range with a mean of In the study by Plescia [ 1991 ] the ridges had a rather high width/height ratio (30-80). For steeper ridges, C will be higher. We consider shortening due to folding as a lower limit for the ridge-! about 0.02 w. Wrinkle ridges that formed as a result of the thrust fault caused by the Meckering earthquake in Western Australia have a typical width of about m and the measured horizontal shortening is typically within 1-2 m [Gordon and Lewis, 1980], that is, about 0.1 w. Thus we can state with confidence that regional strain g is constrained by the limits: O.001w/l < < w/l. The strain is probably within a much narrower range: O.01w/l < < O. l w/l. An estimation of regional strain calculated as O.03w/l for our 30 sites is presented in Table 1. Regional strain varies from site to site within one order of magnitude. The average strain is about 0.3%. Note that the ambiguity of this estimation is very high, at least a factor of 3. Knowing the area of plains with wrinkle ridges on Venus, we can estimate an order of magnitude of global surface shortening due to wrinkle ridge formation. This value is about 0.1% of the planetary surface (if we reference the calculated strain only to those 43% of Venus' surface which Bilotti and Suppe [1997] consider as having prominent wrinkle ridges), or 0.3% (if we reference it to the -70% abundance of plains with wrinkle ridges (Pwr) [Basilevsky and Head, 1995a,b] and consider that post-pwr lavas were mostly overlaid on the Pwr plains while pre-pwr highly deformed units had the same strain as the Pwr plains around). We believe that lower of the estimates (0.1%) is closer to reality. This shortening might be global shrinkage of the surface of the planet that requires an appropriate shortening of the radius of the planet and has serious geophysical implications. But the evident shortening due to formation of wrinkle wridges might also be compensated by the contemporaneous and complementary etensional deformation, which we may not see if these etensional structures are less than the resolution of the Magellan images. In the eample of wrinkle ridges formed as the result of Meckering earthquake, complementary etensional faults and fractures were also formed. They are typically an order of magnitude narrower than the thrust fault ridges. Thus if we were to observe the Meckering area with a resolution lower than 1-2 m, only the compressional ridges could be seen. Thus it is not clear now whether the estimated shortening of the surface of Venus is a real shrinkage of the planet's radius, or is compensated to some etent by complementary etensional deformation. 5. Conclusions Our measurements show that wrinkle ridges on Venus form quasi-periodic, rather than chaotic, networks, similar to the case of the Tharsis region on Mars [Watters, 1991]. The spacing of the wrinkle ridge network on Venus is generally smaller than that in the Tharsis region. This is probably due to the presence of a thinner upper strong crustal layer on Venus compared to that region of Mars. Wrinkle ridges on Venus are systematically narrower than similar features on other terrestrial planets. Regional variations in the typical spacing of the network and wrinkle ridge widths are found on Venus. associated shortening. Knowing the typical width/height The variance of these two characteristics practically the ratios for wrinkle ridges, We can say with confidence that in case of pure folding the shortening is greater than w, where w is ridge width. Shortening due to faulting strongly depends on the fault same (about 50%) but among the 30 sites studied by us there is no correlation between the mean spacings and mean ridge widths of the sites. The values of variance of wrinkle ridge spacing for each of the sites studied on Venus are close to geometry. An evident upper limit for this shortening is the each other and to the values characteristic of the Tharsis ridge width w. Estimations by Plescia [1991] based on a specific model of faulting gave shortening due to faulting of region on Mars. Global surface shortening due to formation of wrinkle ridge network is estimated to be about 0.1%.

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